1 2 Experiments 22S 30 105 Statistical Methods and Computing Recall What is the critical difference between an experiment and an observational study Designing Experiments experimental units individual items on which experiment is done Lecture 8 February 13 2006 usually called subjects when they are human we can measure a response variable individually on each experimental unit Kate Cowles 374 SH 335 0727 kcowles stat uiowa edu treatment a specific experimental condition controlled by the experimenter and applied to the units 3 example agricultural field experiment land available for use in the experiment is divided up into equal sized plots each plot is an experimental unit same variety of corn planted in all plots response variable for each plot is average number of bushels of corn harvested per acre treatments are different types of fertilizers assigned to plots factor a particular explanatory variable manipulated by the experimenter a factor has one or more levels different values that are assigned to different units e g each type of fertilizer in the agricultural example is a different level of the factor fertilizer type 4 A single experiment may involve more than one factor In this case each treatment is defined as the combination of levels of different factors example more complex agricultural field experiment factor A fertilizer type with 3 levels factor B variety of corn with 2 levels then one of 6 possible treatments is assigned to each plot 5 6 The importance of comparison in experiments Example Autism is a severe emotional and developmental disorder that occurs in some children A medical case study reported that an autistic child who received a single injection of a hormone called secretin experience marked improvement in his autism We have no way of knowing what other variables might have influenced the child s autism Comparative experiments are used to separate the effects of an experimental treatment from those of extraneous variables important when we can t control all extraneous variables 7 Groups of subjects in a comparative experiment experimental group s receive treatment s the effects of which are under study control group receives no treatment or a sham treatment Example study reported in Consumer Reports Feb 1976 a group of senior citizens was randomly divided into 2 groups group 1 daily doses of vitamin C group 2 no treatment At end of winter vitamin C group reported fewer colds than no treatment group Investigator concluded that vitamin C helps to prevent colds 8 The placebo effect definition A placebo is a dummy treatment no direct physical effect on response variable In another study decribed in the same Consumer Reports article two treatment groups one group of subjects were given daily vitamin C and told it was a placebo other group received a placebo and were told it was vitamin C The group who thought they were receiving vitamin C reported fewer colds 9 An aside concerning medical studies Note A study of the last mentioned type would be considered unethical today informed consent required for participation in clinical trials For testing new treatments of diseases or conditions for which a treatment already exists the best standard treatment is given to the control group It would be considered unethical to withhold an effective known treatment 11 Randomization 10 Blinding in experiments with human subjects refers to preventing some people involved in the experiment from knowing which subjects are receiving which treatment single blind experiment subjects do not know which treatment they are receiving but study personnel are not blinded double blind experiment neither the subjects nor any study personnel who administer treatment or evaluate response variable know which treatment subjects are receiving 12 Example the Lung Health Study Another aspect of experimental design is how to determine which experimental units receive which treatment clinical trial sponsored by the NIH involving 10 clinical centers in the US and Canada randomization assignment by chance aim to determine the effects on the decline of lung function in smokers already at risk for COPD a lung disease of completely randomized design all experimental units are assigned at random among all the treatments a stop smoking program daily use of an inhaled asthma drug response variable change in FEV1 a measure of lung function from the time a subject entered the study until a followup visit 5 years later subjects approximately 6000 smokers with mild impairment of lung function 13 groups Usual Care group control group received neither the smoking cessation program nor any medication Special Intervention Placebo group received the smoking cessation program but a placebo inhaler Special Intervention Active drug group received the smoking cessation program and the active inhaled drug 15 Blinding in the LHS 14 Was the LHS an experiment or an observational study Was it comparative What were the factors What were the treatments 16 Randomization Patients and study personnel knew who was in UC group assignment of experimental units to treatments based on chance Patients and study personnel knew everyone in both SI groups received smoking cessation program purpose effort to make sure the experimental groups are not systematically different from one another in ways other than the treatment assignment Double blinding as to which SI patients were receiving placebo and which active drug Neither patients clinic personnel nor study directors knew this until end of study in particular subjects are not assigned by the experimenter carried out by computers 17 Completely randomized design 18 Other systems of randomization All the experimental units are allocated at random among all the treatments matched pairs design can be used only if there are only 2 treatments subjects are paired up so each pair is as similar as possible on important known factors that might affect the response variable for each pair randomly as one of the treatments to each subject example if LHS had had a completely randomized design idea would have been put 6000 envelopes in a hat each with a slip of paper inside 2000 saying UC 2000 saying SIP and 2000 saying SIA each time a new patient enrolls in the study draw an envelope at random and have the pharmacists dispense the appropriate treatment 19 randomized block design block a group of experimental units that are known before the experiment to be similar in some way that may affect the